1. Field of the Invention
The present invention relates to an image sensor with an embedded photodiode region and the fabrication method thereof and, more particularly, to an image sensor in which a photodiode region is embedded to extend below a transistor formation region and the fabrication thereof.
2. Description of the Related Art
Image sensors include a CCD (Charge Coupled Device), an APS (Active Pixel Sensor) and a CMOS image sensor as a representative example of APS. CCDs are used in video cameras and so forth and CMOS image sensors are used in low-priced digital still cameras and so forth. Of the two sensors, the CMOS image sensor can be fabricated by means of a CMOS process and has low fabrication costs and is used in battery drive devices such as cellular phones or portable information terminals on account of the small amount of electric power consumed in comparison with CCD image sensors.
A CMOS image sensor comprises a photodiode as an O/E (Opto/Electric) conversion element and extracts incident light intensity as an electrical signal by reading the amount of electrical charge that has accumulated in the photodiode by means of a source follower transistor or the like. CMOS image sensors that are already used are three-transistor-type image sensors that comprise a photodiode, reset transistor, source follower transistor and select transistor. Further, recently, a four-transistor type APS in which a transfer gate transistor is provided between the photodiode and reset transistor has been proposed.
In a four-transistor type APS, a floating diffusion (FD) consisting of a floating diffusion layer is provided at the point of connection between the transfer gate transistor and reset transistor. Further, after the floating diffusion has been brought to the reset level by means of the reset transistor, the electrical charge that has accumulated in the photodiode region is transferred to the floating diffusion and the potential is changed by causing the transfer gate transistor to conduct, and the change in the potential is transferred to the signal line via the source follower transistor. A signal from which noise has been removed can be extracted by detecting the difference between the potential during resetting of the floating diffusion and the potential during the transfer of electrical charge from the photodiode.
Three-transistor-type and four-transistor-type APS appear in Japanese Patent Application Laid Open No. 2002-16243 (published on Jan. 18, 2002), for example.
In addition, a five-transistor-type APS to which an overflow drain transistor is added in order to prevent overflow of the photodiode has also been proposed. By controlling the overflow drain transistor, the integral start time of the photodiode can be controlled and the adoption of a global shutter system is made possible.
Thus, the number of in-pixel transistors increases with an increasingly high performance and the ratio of the surface area to the pixel area of the photodiode region drops, causing a drop in the so-called aperture ratio. In order to solve this problem, sharing of elements such as transistors between adjoining pixels has been proposed.
In addition, in order to prevent a drop in the aperture ratio, a constitution in which a photodiode region is embedded below an in-pixel transistor formation region has also been proposed. For example, such a constitution appears in Japanese Patent Application Laid Open No. 2002-16243 (published on Jan. 18, 2002).
FIG. 1 is a cross-sectional view of the CMOS image sensor that appears in Japanese Patent Application Laid Open No. 2002-16243 (published on Jan. 18, 2002). A gate electrode 55 of a transfer transistor TG, a gate electrode 58 of a reset transistor, and a gate electrode 6l of a source follower transistor are formed via gate oxide films 56, 63, and 64 on a P-type epitaxial layer 52 that is formed on a P-type semiconductor substrate 51, and source and drain regions 57, 59, 60, and 62 are provided on both sides of the gate electrodes. Further, a high-concentration N-type photodiode region 53 is formed in the depth direction from the surface of the epitaxial layer 52 and the photodiode region 53 is embedded to extend below the transfer gate transistor, reset transistor, and source follower transistor. Further, the photodiode region 53 is embedded spaced apart from the surface insulation film 54 by a high-concentration P+ region formed at the surface of the epitaxial layer 52 so that a dark current caused by a leak current from the insulation film 54 can be suppressed.
Thus, in the case of the CMOS image sensor that appears in Japanese Patent Application Laid Open No. 2002-16243, a drop in the aperture ratio is prevented by embedding a photodiode region in overlapping fashion below the in-pixel transistor formation region, whereby the light sensitivity is increased.